Input-dependent synaptic plasticity is critical for the reproducible activation of a specific neuronal assembly encoding a particular memory. The synaptic tagging hypothesis, which suggests how input specificity is maintained in late-phase synaptic plasticity, attempts to explain the persistence of long-term memory. However, it has been difficult to identify proteins that behave as the hypothesis predicts. Okada et al. investigated whether the regulated spine entry of a late-phase-related somatically synthesized plasticity-related protein, Vesl-1S, works as a synaptic tag. Vesl-1S protein was carried from the soma to every dendrite and recruited into spines by synaptic activation in an input-specific manner. Spine entry was protein-synthesis independent, was NMDA-receptor dependent, and had a persistent lifetime of activation. These results provide long-sought evidence for the input-specific capturing of a plasticity-related protein as postulated by the synaptic tagging hypothesis.